I personally believe it to be rather silly that a guy in full plate can carry around a large two handed weapon / polearm while hauling around another one on their back. Even worse, they run around with finesse and dance around and feint with no issues. Realistically (and maybe this belongs in Realism Discussion, but I think it fits balance a bit more) this should be impossible, if not extremely difficult and debilitating.
I propose that a few more weapons become 'unsheathable', preferably weapons of ridiculous length and weight that are just too bulky to be carried on the back. Most of the poleaxes, and even a few of the greatswords just look absurd when placed on someone's back, and if you watch the above video, you can see that even with a katana style weapon he has a hard time unsheathing it. Imagine if he tried it with something the same length as the Danish Greatsword.
Just a suggestion, it's already been done with all of the lances, the flamberge, and a few other two directional polearms. Like I said above, this kinda goes for realism discussion, but at the same time it heavily effects gameplay.
He brings up a few more points above, including a solution to the whole "unsheathable from the back" thing, but also debunks that with another point.
P.S. let us slide on our horses, my signature should show what I mean.